A porous sintered material of polyolefin resin particles having continuous pores has excellent characteristics such as fluid filtration function, permeability function and induction function and is widely utilized as a filter, an aeration tube, an induction material for liquid and the like. Specifically, a hydrophilic porous sintered material mainly comprising hydrophilized polyethylene is utilized for uses such as absorption, diffusion, transpiration, permeation, induction of water and the like, utilizing its absorption functions. For example, a polyolefin sintered material described in PATENT DOCUMENT 1 has been out into practical use in a material for absorbing dew condensation water in the vegetable compartment, chilled compartment or the like of the refrigerator and a humidifier in the refrigerator, a humidification element of the conditioner or the like, an ink absorber in the printer, and the like.
Porous materials having water-absorbing functions include, besides a hydrophilic polyolefin porous sintered material, cloth and nonwoven fabric of hydrophilized synthetic fiber, asbestos cloth or natural fiber textile, fabric such as paper, hollow fiber membrane and microporous membrane of hydrophilized thermoplastic resin, sintered material of metal and ceramics, water-absorbent polymeric material, and the like. Since the hydrophilic polyolefin porous sintered material comprises all advantages such as the very small change in dimension by swelling upon absorbing water or the like, maintaining high strength as a compact, and being capable of coping with complicated shapes, the development in various fields such as electronics and biotechnology has been expected.
In order to meet the above expectations, it is important sustain water-absorbing functions, and some methods have been suggested in the past.
PATENT DOCUMENT 2 and PATENT DOCUMENT 3 describe a sintered material of polyethylene resin particles whose particle surface is sulfonated. However, due to severe production method comprising the step of sulfonating with fuming sulfuric acid, hot concentrated sulfuric acid or the like, by-product accompanied with oxidation, dehydration or the like is much generated, and further, it is problems that discoloration and deterioration of polyolefin resin are caused and it is a very hazardous chemical reaction.
PATENT DOCUMENT 4 describes a sintered material obtained by introducing a grafted polymer chain into polyolefin resin particles and thereafter substituting a functional group on the surface. However, in the above method, since the grafted polymer chain does not have hydrophilicity, water-absorbing functions are not found in the sintered material, and water cannot be passed through pores in the sintered material unless applying pressure.
PATENT DOCUMENT 5 describes a method of hydrophilizing a sintered material, comprising the steps of providing hydrophilicity by treating the surface of a sintered material with oxygen plasma or the like, and thereafter applying a coating material thereon. In this method, adhesion of the coating material is increased by high-energy treatment. However, since the coating material itself is not a material that can withstand severe conditions such as hot water treatment or the like, sustainability of water-absorbing functions is not increased.
In addition to the above, as a method of hydrophilizing polymeric materials such as hollow fiber and microporous membrane, for example, as described in PATENT DOCUMENT 6, PATENT DOCUMENT 7 and the like, a method of grafting a functional group by ionizing radiation is suggested. However, all polymeric materials exemplified the above documents are practically thin films, so that it is very difficult to spontaneously suction and hold water utilizing capillary action and repeatedly use water while maintaining rigidity and dimensional stability upon absorbing water as a compact. In addition, even though water is absorbed, water easily seeps out only by gently pressing the polymeric material. Therefore, upon practically utilizing the above polymeric materials, an insufficient property must be compensated by complexing with other materials. However, since the junction of materials are at high risk of being structural defect, substantial effort and cost are required for complexing, and it ends up going through very inefficient step in the industrial process.
With the development of industry using functional composite materials, expectations for a hydrophilic polyolefin sintered material as a polymeric material having plural functions are further being raised. However, despite being utilized as a compact through the ages, specific examples for sustaining water-absorbing functions of the porous sintered material of polyolefin resin particles have not yet been shown. This is because the sintered material has comparatively large pores as a porous material and is a thick compact, so that even very minor water-absorbing defect ends up having an enormous influence on bulky water-absorbing functions. Therefore, for the realization of hydrophilic polyolefin sintered material having both long-lasting sustainability of water-absorbing functions and low dissolubility, problems in both improvement in water-absorbing functions and basic structure as a compact need to be solved.    PATENT DOCUMENT 1: JP-B-H04-28021    PATENT DOCUMENT 2: JP-B-H03-28457    PATENT DOCUMENT 3: JP-B-H03-63577    PATENT DOCUMENT 4: WO2003-14205    PATENT DOCUMENT 5 JP-A-2005-510608    PATENT DOCUMENT 6: JP-A-H05-156057    PATENT DOCUMENT 7: JP-A-2004-35582